Radio communication systems generally provide two-way voice and data communication between remote locations. Examples of such systems are cellular and personal communication system (“PCS”) radio systems, trunked radio systems, dispatch radio networks, and global mobile personal communication systems (“GMPCS”) such as satellite-based systems. Communication in these systems is conducted according to a pre-defined standard. Mobile devices or stations, also known as handsets, portables or radiotelephones, conform to the system standard to communicate with one or more fixed base stations. It is important to determine the location of such a device capable of radio communication especially in an emergency situation. In addition, the United States Federal Communications Commission (“FCC”) has required that cellular handsets must be geographically locatable by the year 2001. This capability is desirable for emergency systems such as Enhanced 911 (“E-911”). The FCC requires stringent accuracy and availability performance objectives and demands that cellular handsets be locatable within 100 meters 67% of the time for network based solutions and within 50 meters 67% of the time for handset based solutions.
Current generations of radio communication generally possess limited mobile device location determination capability. In one technique, the position of the mobile device is determined by monitoring mobile device transmissions at several base stations. From time of arrival or comparable measurements, the mobile device's position may be calculated. However, the precision of this technique may be limited and, at times, may be insufficient to meet FCC requirements. For example, providers of wireless communication services may have installed mobile device location capabilities into their networks. In operation, these network overlay location systems take measurements on radio frequency (“RF”) transmissions from mobile devices at base station locations surrounding the mobile device and estimate the location of the mobile device with respect to the base stations. Because the geographic location of the base stations is known, the determination of the location of the mobile device with respect to the base station permits the geographic location of the mobile device to be determined. The RF measurements of the transmitted signal at the base stations may include the time of arrival, the angle of arrival, the signal power, or the unique/repeatable radio propagation path (radio fingerprinting) derivable features. In addition, these location systems may also use collateral information, e.g., information other than that derived for the RF measurement to assist in the geographic location (“geolocation”) of the mobile device, i.e., location of roads, dead-reckoning, topography, map matching, etc.
In a network-based geolocation system, the mobile device to be located may be typically identified and radio channel assignments determined by (a) monitoring the control information transmitted on radio channel for telephone calls being placed by the mobile device or on a wireline interface to detect calls of interest, i.e., 911, and/or (b) a location request provided by a non-mobile device source, i.e., an enhanced services provider. Once a mobile device to be located has been identified and radio channel assignments determined, a location determining system is first tasked to determine the geolocation of the mobile device and then directed to report the determined position to the requesting entity or enhanced services provider. The monitoring of the RF transmissions from the mobile device or wireline interfaces to identify calls of interest is known as “tipping”, and generally involves recognizing a call of interest being made from a mobile device and collecting the call setup information. Once the mobile device is identified and the call setup information is collected, the location determining system can be tasked to geolocate the mobile device.
In another technique, a mobile device may be equipped with a receiver suitable for use with a Global Navigation Satellite System (“GNSS”) such as the Global Positioning System (“GPS”). GPS is a radio positioning system providing subscribers with highly accurate position, velocity, and time (“PVT”) information. With GPS, signals from a constellation of satellites arrive at a GPS receiver and are utilized to determine the position of the receiver. GPS position determination is made based on the time of arrival (“TOA”) of various satellite signals. Each of the orbiting GPS satellites broadcasts spread spectrum microwave signals encoded with satellite ephemeris information and other information that allows a position to be calculated by the receiver. Presently, two types of GPS measurements corresponding to each correlator channel with a locked GPS satellite signal are available for GPS receivers. The two carrier signals, L1 and L2, possess frequencies of 1.5754 GHz and 1.2276 GHz, or wavelengths of 0.1903 m and 0.2442 m, respectively. The L1 frequency carries the navigation data as well as the standard positioning code, while the L2 frequency carries the P code and is used for precision positioning code for military applications. The signals are modulated using bi-phase shift keying techniques. The signals are broadcast at precisely known times and at precisely known intervals and each signal is encoded with its precise transmission time.
GPS receivers measure and analyze signals from the satellites, and estimate the corresponding coordinates of the receiver position, as well as the instantaneous receiver clock bias. GPS receivers may also measure the velocity of the receiver. The quality of these estimates depends upon the number and the geometry of satellites in view, measurement error and residual biases. Residual biases generally include satellite ephemeris bias, satellite and receiver clock errors and ionospheric and tropospheric delays. If receiver clocks were perfectly synchronized with the satellite clocks, only three range measurements would be needed to allow a user to compute a three-dimensional position. This process is known as multilateration. However, given the engineering difficulties and the expense of providing a receiver clock whose time is exactly synchronized, conventional systems account for the amount by which the receiver clock time differs from the satellite clock time when computing a receiver's position. This clock bias is determined by computing a measurement from a fourth satellite using a processor in the receiver that correlates the ranges measured from each satellite. This process requires four or more satellites from which four or more measurements can be obtained to estimate four unknowns x, y, z, b. The unknowns are latitude, longitude, elevation and receiver clock offset. The amount b, by which the processor has added or subtracted time, is the instantaneous bias between the receiver clock and the satellite clock. It is possible to calculate a location with only three satellites when additional information is available. For example, if the elevation of the handset or mobile device is well known, then an arbitrary satellite measurement may be included that is centered at the center of the earth and possesses a range defined as the distance from the center of the earth to the known elevation of the handset or mobile device. The elevation of the handset may be known from another sensor or from information from the cell location in the case where the handset is in a cellular network.
Traditionally, satellite coordinates and velocity have been computed inside the GPS receiver. The receiver obtains satellite ephemeris and clock correction data by demodulating the satellite broadcast message stream. The satellite transmission contains more than 400 bits of data transmitted at 50 bits per second. The constants contained in the ephemeris data coincide with Kepler orbit constants requiring many mathematical operations to turn the data into position and velocity data for each satellite. In one implementation, this conversion requires 90 multiplies, 58 adds and 21 transcendental function cells (sin, cos, tan) in order to translate the ephemeris into a satellite position and velocity vector at a single point, for one satellite. Most of the computations require double precision, floating point processing.
Thus, the computational load for performing the traditional calculation is significant. The mobile device must include a high-level processor capable of the necessary calculations, and such processors are relatively expensive and consume large amounts of power. Portable devices for consumer use, e.g., a cellular phone or comparable device, are preferably inexpensive and operate at very low power. These design goals are inconsistent with the high computational load required for GPS processing.
Further, the slow data rate from the GPS satellites is a limitation. GPS acquisition at a GPS receiver may take many seconds or several minutes, during which time the receiver circuit and processor of the mobile device must be continuously energized. Preferably, to maintain battery life in portable receivers and transceivers such as mobile cellular handsets, circuits are de-energized as much as possible. The long GPS acquisition time can rapidly deplete the battery of a mobile device. In any situation and particularly in emergency situations, the long GPS acquisition time is inconvenient.
Assisted-GPS (“A-GPS”) has gained significant popularity recently in light of stringent time to first fix (“TTFF”), i.e., first position determination, and sensitivity, requirements of the FCC E-911 regulations. In A-GPS, a communications network and associated infrastructure may be utilized to assist the mobile GPS receiver, either as a standalone device or integrated with a mobile station or device. The general concept of A-GPS is to establish a GPS reference network (and/or a wide-area D-GPS network) including receivers with clear views of the sky that may operate continuously. This reference network may also be connected with the cellular infrastructure, may continuously monitor the real-time constellation status, and may provide data for each satellite at a particular epoch time. For example, the reference network may provide the ephemeris and the other broadcast information to the cellular infrastructure. In the case of D-GPS, the reference network may provide corrections that can be applied to the pseudoranges within a particular vicinity. As one skilled in the art would recognize, the GPS reference receiver and its server (or position determining entity) may be located at any surveyed location with an open view of the sky.
However, the signal received from each of the satellites may not necessarily result in an accurate position estimation of the handset or mobile device. The quality of a position estimate largely depends upon two factors: satellite geometry, particularly, the number of satellites in view and their spatial distribution relative to the user, and the quality of the measurements obtained from satellite signals. For example, the larger the number of satellites in view and the greater the distances therebetween, the better the geometry of the satellite constellation. Further, the quality of measurements may be affected by errors in the predicted ephemeris of the satellites, instabilities in the satellite and receiver clocks, ionospheric and tropospheric propagation delays, multipath, receiver noise and RF interference. Therefore, the improvements offered by A-GPS do not guarantee location in all environments; rather, A-GPS merely offers an improvement over conventional GPS.
The aforementioned shortcomings of the prior art has led the industry to pursue alternative location methods as a backup solution for a primary location methodology such as, but not limited to, A-GPS. One such method may generally be referred to as Network Measurement Report (“NMR”) location methods. This particular location methodology generally attempts to locate mobile devices based on the normal network measurements made by the handset that are periodically provided back to the network. One characteristic when utilizing NMRs for location of a mobile device is that typically a large amount of data must be provided to Position Determining Equipment (“PDE”) to produce a location estimate with satisfactory accuracy. In the event that many wireless subscribers are being located, the volume of data passed from the communications network to the PDE may be very large and unmanageable.
There are several methods to communicate the NMR back to the PDE from the network. One non-limiting method may be through the existing communication links established between base transceiver stations (“BTS”) or base stations and a Serving Mobile Location Center (“SMLC”) that serve as a path for information transfer for A-GPS positioning. FIG. 1 is an illustration of an exemplary architectural diagram for a communications system. A mobile station or mobile device 101 may be in communication with a BTS 105 via a wireless interface Um. The base station controller (“BSC”) 107 manages radio resources including the BTS 105 via an Abis interface. The Abis interface is generally an open interface defined as part of the ETSI specification for GSM and carries call set up information, including voice channel assignments between the BSC 107 and BTS 105. A Mobile switching center/visitor location register (“MSC/VLR”) 113 may coordinate between the mobile appliance communication network and a global mobile location center (“GMLC”) 117. In operation, one or more location measurement units (“LMUs”) 103 may be operably connected to a PDE 121 or BTS 105 which may also be operably connected to the BSC 107 via the Abis wire line interface. The LMUs 103 may make measurements on the RF signals of the Um interface, along with other measurements to support one or more of the positioning methods well known in the art. The measurements from the LMUs may be provided to a servicing mobile location center (“SMLC”) 109 via the BSC 107 where the location of the mobile device 101 may be determined. The SMLC 109 may also be operably connected to the MSC/VLR 113 via an Ls interface. The GMLC 117 may be connected to a home location register (“HLR”) 111 over an Lh interface and the MSC/VLR 113 over an Lg interface. Of course, the GMLC 117 may be operably connected to a location based services (“LBS”) network 119 over an Le interface. A global mobile switching center (“GMSC”) 115 may also be operably connected to the MSC/VLR 113.
In a GSM or Integrated Digital Enhanced Network (“IDEN”) network, exemplary communication links that establish a path from the BSC 107 to the SMLC 109 are the “A” and “Ls” interfaces. These interfaces generally carry digital traffic over an SS7 connection which may be physically carried by some number of T1 style communication links in the underlying wired telecommunications network. In the event that NMR location is needed for a wireless provider, there may potentially be a substantial increase in the amount of data that the A and Ls interfaces must support. While expansion of the capacity of these interfaces is an option for resolving the problem, this may be a very expensive solution for a wireless carrier to support, given that the solution may require a significant nationwide capacity increase. Therefore, there is a need in the art to implement alternative communication methods for which NMR location could be supported in a wireless network that would not substantially increase implementation and operational costs.
Further, there is a need to provide a process to efficiently and effectively handle the vast amount of data being sent between a wireless communications network and the large number of mobile devices for which locations are to be determined. In this regard, embodiments of the present subject matter can overcome the limitations of the prior art by estimating the location of a mobile device using, at least in part, one or more Network Measurement Reports (“NMRs”) which may include measurement data for a number of locations within a geographic region.
Accordingly, there is a need for a method and apparatus for determining the location of a mobile device that would overcome the deficiencies of the prior art. Therefore, an embodiment of the present subject matter provides a method for determining an approximate location of a mobile device. The method may comprise the steps of determining at a first node of a network the occurrence of a predetermined event and storing at a second node of the network measurement data associated with the mobile device. One or more attempts may be made to determine a location of the mobile device using a first location methodology. Upon failure of the location attempt, mobile device data may be received at a third node of the network from the second node. An approximate location of the mobile device using the mobile device data may then be determined at the third node. In another embodiment of the present subject matter, the method may comprise sending the determined approximate location to a fourth node of the network.
Another embodiment of the present subject matter provides a system for determining an approximate location of a mobile device. The system may comprise circuitry for determining at a first node of a network the occurrence of a predetermined event, and a database at a second node of the network for storing data associated with the mobile device. The system may also comprise a processor for attempting to determine a location of the mobile device, and a receiver at a third node of the network for receiving the mobile device data from the second node upon failure of the location attempt. The system may include circuitry for determining at the third node an approximate location of the mobile device using the mobile device data. In another embodiment of the present subject matter the system may further comprise circuitry for sending the determined approximate location to a fourth node of the network.